Flexible, air-permeable plastic sheet

ABSTRACT

Here is disclosed a flexible and air-permeable plastic sheet comprising a plurality of first direction ribs (Y 1 , Y 2 , . . . ) each having opposite sides (2, 3) extending in the first direction (Y) and curved downward crossing a plurality of second direction ribs (X 1 , X 2 , . . . ) each having opposite sides (4, 5) extending in the second direction (X) and curved downward so that each pair of adjacent first direction ribs and each pair of adjacent second direction ribs crossing these first direction ribs define together an air-permeable vacant space. At each crossing (C 1 , C 2 , C 3 , C 4 ), one of the ribs bulges upward and opposite side edges of this rib crosses the top (T 1 , T 2 ) of the other rib. The plastic sheet of this invention is featured by less gloss and less sticky touch, and preferably used as a topsheet of a disposable hygienic wearable article.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a flexible, air-permeable plastic sheethaving an appearance like woven fabric.

BACKGROUND OF THE INVENTION

It is well known to provide a flexible thermoplastic sheet inclusive ofplastic film with a plurality of fine pores and thereby to make thesheet air-permeable. The porous plastic sheet formed in this mannerusually has slimy gloss and sticky touch characterizing the plasticsheet which are disliked by users in some applications of the sheet. Toalleviate such gloss and to improve the touch of the sheet, severaltechniques have already been proposed, for example, the technique bywhich the sheet surface is embossed to take the gloss off andsimultaneously to provide the surface with irregularities and pores, andthe technique by which a third ingredient having delustering effect aswell as pore forming effect is previously mixed into raw material forthe sheet so that desired delustering and pore forming effects may beachieved on a step of sheet production.

However, these well known techniques have not been able to achieve asatisfactory improvement of the plastic sheet particularly for suchplastic sheet used as a topsheet of disposable hygienic wearablearticles such as a sanitary napkin and a disposable diaper, because saidgloss and touch of the topsheet of such hygienic wearable articles,which are destined to be in direct contact with wearer's skin, aredecisively disliked. U.S. Pat. No. 4,342,314 discloses a technique toprovide a flexible plastic sheet with a fibrous appearance and capillarystructure. This technique also intends to alleviate the sticky touchusually characterizing the plastic sheet, but the sheet obtained by thistechnique is considered to have a smooth surface and rather slimy glossbecause of its smoothness.

Accordingly, it is a principal object of the present invention toalleviate said gloss and undesirable touch conventionally characterizingthe plastic sheet by providing the plastic sheet with air-permeablevacant spaces defined by ribs crossing one another so that these ribsand vacant spaces may cooperate together to create an appearance likewoven fabric.

DISCLOSURE OF THE INVENTION

The object set forth above is achieved, according to the presentinvention, by flexible, air-permeable plastic sheet having an appearancelike woven fabric, said plastic sheet comprising:

a plurality of ribs extending in a first direction, each having oppositeside edges curved downward and a plurality of ribs extending in a seconddirection, each having opposite side edges curved downward so that thesefirst and second direction ribs cross one another to present saidappearance like woven fabric;

each pair of adjacent said first direction ribs and each pair ofadjacent said second direction ribs crossing these first direction ribsdefining together an air-permeable vacant space; and

said pair of first direction ribs bulge upward at first and secondcrossings which are adjacent and diagonally opposed to each other sothat bulgings aligned in the first direction are formed and a pair oflower edges of each said bulging are connected to tops of said pair ofsecond direction ribs, on one hand, and said pair of second directionribs bulge upward at third and fourth crossings which are adjacent anddiagonally opposed to each other so that bulgings aligned in the seconddirection are formed and a pair of lower edges of each said bulging areconnected to tops of said pair of first direction ribs, on the otherhand.

According to an aspect of the invention, each of said first and seconddirection ribs preferably has a hollow arcuate cross-section.

According to another aspect of the invention, each said second directionrib has preferably a width smaller than a width of each said firstdirection rib at least along a longitudinally intermediate portion ofsaid second direction rib.

According to still another aspect of the invention, said pair ofadjacent fist direction ribs are preferably provided with bridgesconnecting opposite lower edges of these ribs.

Advantageous effect provided by the invention will be readily apparentfrom the following description.

In the plastic sheet of the invention constructed as has been mentionedabove, the first direction ribs cross the second direction ribs and, ateach crossing, one of ribs bulges upward so that each rib hasundulations between each pair of ribs crossing this rib and creates anappearance corresponding to a texture of plain weave fabric. Provisionof said bridges between each pair of adjacent ribs creates an appearancecorresponding to a texture of twill weave fabric.

A hollow arcuate cross-section of each rib provides the rib with acushioning effect and thereby provides the entire plastic sheet with acomfortable bulky touch.

The first and second direction ribs are arranged at different pitches,respectively, to complicate the rib arrangement and thereby to create acomplicated appearance of the plastic sheet like woven fabric.

Complicated variation of the plastic sheet surface given by bulgings andundulations of each rib promotes diffused reflection of incident lightand thereby effectively alleviates the slimy gloss conventionallycharacterizing the plastic sheet in general.

Bulgings and undulations on the surface of the plastic sheet effectivelyreduce the area of the sheet destined to be in contact with wearer'sskin and thereby alleviate the sticky touch also conventionallycharacterizing the plastic sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 6 are scale-enlarged perspective, plan and sectionalviews partially showing an embodiment of a flexible, air-permeableplastic sheet constructed according to the teachings of the invention;

FIGS. 7 and 8 are scale-enlarged fragmentary plan views showingalternative embodiments of such plastic sheet;

FIGS. 9 through 11 are scale-enlarged fragmentary plan and sectionalview showing still another embodiment of such plastic sheet;

FIG. 12 is a scale-enlarged fragmentary plan view showing furtheranother embodiment of such plastic sheet; and

FIG. 13 is a perspective view partially broken away, showing the plasticsheet of the invention exemplarily used on a sanitary napkin.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 through 6, an embodiment of the flexible,air-permeable plastic sheet 1 constructed in accordance with theteachings of the invention is partially illustrated in plan andsectional views, respectively.

Referring to FIG. 1, solid lines represent the minimum unit of the sheet1 comprising countless ribs and this unit is repeated in longitudinaland transverse directions to form a complete sheet 1. This unit of thesheet 1 comprises a pair of ribs Y₁, Y₂ extending in a first direction Yand a pair of ribs X₁, X₂ extending in a second direction X, these ribsbeing substantially uniform in their widths. The respective ribs aretransversely curved downwardly of the sheet 1 so that the ribs Y₁, Y₂have respective pairs of lower edges 2, 3 longitudinally extending inparallel to each other, respectively, and the ribs X₁, X₂ similarly haverespective pairs of lower edges 4, 5 longitudinally extending inparallel to each other, respectively. The pair of adjacent ribs Y₁, Y₂extending in the first direction cross the pair of adjacent ribs X₁, X₂extending in the second direction at crossings C₁ through C₄. At thefirst and second crossings C₁, C₂ which are adjacent and diagonallyopposed to each other, the ribs Y₁, Y₂ of the first direction bulgeupward to form bulgings R₁, R₂, respectively. Similarly, the ribs X₁, X₂of the second direction bulge upward to form bulgings R₃, R₄,respectively, at the third and fourth crossings C₃, C₄ which areadjacent and diagonally opposed to each other. At the bulgings R₁, R₂,the respective pairs of lower edges 2, 3 of the first direction ribs Y₁,Y₂ are connected to respective tops T₂ of the second direction ribs X₁,X₂ which extend in the second direction. Similarly, at the bulgings R₃ ,R₄, the respective pairs of lower edges 4, 5 of the second directionribs X₁, X₂ are connected to the respective tops T₁ of the firstdirection ribs Y₁, Y₂ which extend in the first direction. The ribs Y₁,Y₂ and the adjacent ribs X₁, X₂ meet substantially at right angles andthese four ribs define together an air-permeable vacant space 6, whichis a through hole. The sheet 1 constructed in this manner has anappearance corresponding to that of plain weave fabric.

FIG. 2 is a plan view overlooking the sheet 1 shown by FIG. 1. Referringto this figure, the first direction ribs Y₁, Y₂ have a pitch P₁ and thesecond direction ribs X₁, X₂ have a pitch P₂, wherein P₁ <P₂. However,the sheet 1 may be constructed so as to extablish a relationship of P₁≧P₂. In addition, the ribs may obliquely cross one another, instead ofcrossing at right angles.

The ribs Y₁, Y₂ have a width d₁ and the ribs X₁, X₂ have a width d₂.While this specific embodiment is illustrated as the width d₁ issubstantially equal to the width d₂, any of these d₁ and d₂ may belarger than the other and each rib may be locally thinned along itslongitudinally intermediate portion.

FIG. 3 is a schematic sectional view commonly showing the rib Y₁ along aline III--III and the rib X₂ along a line III'--III' in FIG. 2. Asshown, each rib has a substantially semi-circular cross-sectionconsisting of the top T₂ or T₁, a substantially uniform thickness t, andthe pair of tapered lower edges 2, 3 (on the rib Y₁) or 4, 5 (on the ribX₂).

Referring to FIG. 4, the first direction rib Y₁ is shown at the crossingC₁ in a section taken along a line IV--IV in FIG. 2 extending at anangle of 45° with respect to this rib Y₁. The rib Y₁ presents asubstantially semi-elliptical cross-section including the top T₁, thethickness t and a pair of tapered lower edges 12, 13 along which the topT₂ and the vicinity thereto are connected to the lower edges 2, 3 of therib Y₁. The cross-sections taken along lines extending at an angle of45° with respect to the first direction at the other crossings C₂through C₄ are substantially identical to that shown by FIG. 4 exceptthe respective ribs defining the bulgings.

FIG. 5 is a sectional view taken along a line V--V in FIG. 2, andincludes sections of the bulgings R₁, R₄ at the crossings C₁, C₄,respectively. At the crossing C₁, the rib Y₁ bulges upward to form thebulging R₁ and the lower edge 2 thereof is connected to the top T₂ ofthe rib X₂. At the opposite sides of the bulging R₁, it will be seen inFIG. 5 that the lower edges 4, 5 of the rib X₂ are connected throughcontours 10 to the lower edge 2 of the rib Y₁. At the crossing C₄, therib X₁ bulges upward to form the bulging R₄ and the lower edges 4, 5thereof are connected to the top T₁ of the rib Y₁. It will be apparentalso that the rib Y₁ has undulations between the ribs X₁, X₂.

FIG. 6 is a sectional view taken along a line VI--VI in FIG. 2, andincludes sections of the bulgings R₁, R₃ at the crossings C₁, C₃,respectively. The manner in which the ribs Y₁ and X₂ bulge upward andare connected to each other at the crossing C₁ has already beendescribed in reference with FIG. 5. At the crossing C₃, the rib X₂bulges upward to form the bulging R₃ and the lower edge 5 thereof isconnected to the top T₁ of the rib Y₂. At the opposite sides of thebulging R₃, the lower edges 2, 3 of the rib Y₂ are connected throughcontours 11 to the lower edges 5 of the rib X₂.

Sections taken along lines VII--VII and VIII--VIII in FIG. 2,respectively, substantially correspond to FIGS. 5 and 6 laterallyreversed, respectively and are not shown for simplicity.

In order that the sheet 1 can have an appearance of woven fabric, therib width d₁, d₂ should be 0.01 to 3 mm, more preferably, 0.05 to 2 mmand the number of vacant spaces 6 should be 5 to 90 per 25.4 mm, morepreferably, 10 to 60 per 25.4 mm of the respective ribs. The sheet 1 maybe made from 5 to 50 g/m², more preferably, 20 to 30 g/m² ofpolyethylene, polypropylene, polyester or other thermoplastic sheets.Improvement of gloss as well as touch and further emphasis of fibrousappearance are effectively achieved by various measures such asrough-surface finishing of the sheet 1 for better diffused reflection ofincident light, contouring of each vacant space 6 by irregular curve,irregular variation of the rib width longitudinally thereof, appropritevariation of the rib pitch and appropriate coloring of the sheet 1.

Referring to FIG. 7, an alternative embodiment of the invention is shownin a plan view. As illustrated, each rib Y₁, Y₂ has a width d₁substantially equal to a width d₂ of each rib X₁, X₂ and said rib Y₁, Y₂has a pitch P₁ substantially equal to a pitch P₂ of said rib X₁, X₂.

Referring to FIG. 8 further alternative embodiment of the invention isshown also in a plan view. In this specific plastic sheet 1, the widthsd₂ of the respective ribs X₁, X₂ continuously vary between the crossingsC₁ and C₃ and between the crossings C₂ and C₄, respectively. Morespecifically, the ribs X₁, X₂ have the most reduced widths d₄ at themiddle points, respectively.

Referring to FIGS. 9 through 11, still another embodiment is shown inplan and sectional views, respectively. As seen in FIG. 9, this plasticsheet 1 includes a bridge 15 connecting the ribs Y₁, Y₂ to each other.FIGS. 10 and 11 are sectional views taken along lines IX--IX and XI--XIin FIG. 9. The bridge 15 extends between the lower edge 3 of the rib Y₁and the opposed lower edge 2 of the rib Y₂, and has a width W and athickness t'. The width W may irregularly vary between the opposite endsof the bridge 15 and preferably the thickness t' is substantially equalto or smaller than the thickness t of the rib so that the desiredflexibility of the sheet 1 may be maintained. It is unnecessary toprovide all the vacant spaces 6 with such bridges 15. In this specificembodiment, the bridge 15 presents together with the adjacent ribs anappearance corresponding to that of twill weave fabric.

Referring to FIG. 12, further another embodiment is shown in a planview. The ribs Y₁, Y₂, Y₃ have pitches P₁, P₃ in the first directionwhile the ribs X₁, X₂, X₃ have pitches P₂, P₄, and these pitches aredifferent from one another. In other words, the ribs of the firstdirection (Y₁, Y₂, Y₃ . . . ) are arranged at the alternatively repeatedpitches P₁, P₃ and the ribs of the second direction (X₁, X₂, X₃ . . . )are arranged at the alternately repeated pitches P₂, P₄ to form thecomplete sheet 1.

The plastic sheet 1 may be produced by various methods and one ofrelatively simple methods is based on a principle of thermoformingcomprising steps of placing a plastic sheet, while it is thermallysoftened, on a top of a wire net which has a texture like plain weave,twill weave, satin weave or the like formed by wire, subjecting theplastic sheet to an effect of vacuum suction provided from an undersideof the wire net, and providing said plastic sheet with pores atlocations corresponding to meshes of the wire net by rupturing theplastic sheet at said locations under the effect of vacuum suction. Forcontinuous production of the plastic sheet 1, a rotatable drum may beprovided around its outer peripheral surface with the wire net, saidrotatable drum may be adapted to be subjected to the effect of vacuumsuction at predetermined locations on said outer peripheral surface anda roll of material sheet may be continuously dereeled, while the sheetis thermally softened, onto said rotatable drum. The plastic sheet 1unwound from the drum may be then transversely spread to enlarge thevacant spaces of the pores defined by the ribs, to enlarge the ribwidths or to obtain the irregularly varying rib widths. For example, thesheet 1 shown by FIG. 8 may be obtained by spreading the sheet 1 shownby FIG. 1. The sheet may be previously softened by heating, if necessaryfor said step of spreading. Such method often results in the sheet 1having a locally uneven configuration and it should be understood thatsuch sheet 1 is also covered by the scope of the invention.Additionally, the wire net may be at least partially formed by strandedwire and the sheet may be pressed against such wire net under the effectof vacuum suction to obtain fine irregularities on the rib surfaceswhich have been intimate with said stranded wire during saidthermoforming.

Referring to FIG. 13, a sanitary napkin 20 utilizing the plastic sheet 1as a topsheet 21 is shown in a perspective view partially broken away.The napkin 20 comprises an absorbent core 22 adapted to absorb and holdmenstrual discharge, an air- and liquid-permeable topsheet 21 and aliquid-impermeable backsheet 23. The topsheet 21 comprises the plasticsheet 1 shown by FIG. 1, the absorbent core 22 comprises a mixture offluffy pulp and high water absorption polymer powder and the backsheet23 comprises polyethylene film. The topsheet 21 and the backsheet 23 arebonded together along four edges of the napkin 20 so as to hold backmenstrual discharge. The lower edges 2 through 5 of the ribsconstituting the plastic sheet 1 are inwardly directed and contactedwith the absorbent core 22.

Industrial Usefulness

The plastic sheet constructed in accordance with the teachings of thepresent invention is industrially useful from the following viewpoints:

1) The sheet has fine bulgings and undulations on its surface on whichthe diffused reflection of incident light rays occurs to alleviatesomewhat slimy gloss conventionally characterizing plastic sheets ingeneral and which reduce an area of the sheet destined to be in contactwith a wearer's skin so as to eliminate somewhat sticky touch alsoconventionally characterizing plastic sheets in general.

2) The sheet presents a surface appearance resembling a texture of wovenfabric such as plain weave, twill weave or the like, depending on thepattern of rib arrangement.

3) Each rib is hollow one having the semi-circular cross-section whichcontributes to provide cushiony and bulky touch.

4) When the sheet of this invention is used as a liquid-permeabletopsheet for a disposable hygienic wearing article such as sanitarynapkin, the sheet provides a comfortable touch and body fluidsdischarged onto this sheet will rapidly flow down along the ribs havingsemi-circular cross-sections through the vacant spaces into an absorbentcore so as to prevent the body fluids from stagnating in said topsheet.

We claim:
 1. A flexible, air-permeable plastic sheet comprising(a) aplurality of first ribs extending in parallel in a first direction, eachfirst rib having opposite side edges curved in a downward direction, (b)a plurality of second ribs extending in parallel in a second directionthat is generally perpendicular to said first direction, each of saidsecond ribs having opposite side edges curved in a downward direction,(c) said first ribs and said second ribs intersecting one another atspaced apart intervals in an over and under sequence to present theappearance of a woven fabric that contains air-permeable vacant spaces;(d) said first ribs and said second ribs bulging upwardly at theirpoints of intersection, (e) said first ribs being drawn between theirpoints of intersection with said second ribs so that the portion of saidfirst ribs between said points of intersection significantly narrows inwidth and is narrower in width than the width of said second ribsbetween said points of intersection.